Approximately 16 million people (roughly 6% of the population) in the United States suffer from diabetes. Diabetes is the seventh leading cause of death (sixth leading cause of death by disease) in the United States claiming approximately 200,000 lives each year. Moreover, diabetes is one of the most costly health problems in America, running upwards of $92 billion in health care costs annually. Life-threatening complications associated with diabetes include cardiovascular disease and stroke, high blood pressure, blindness, kidney disease, nerve disease and amputation. Of the 16 million diabetics in the United States, approximately 5-10% suffer from IDDM (Insulin-Dependent Diabetes Mellitus) otherwise known as Type 1 diabetes. At least 30,000 new cases of IDDM are diagnosed each year. Persons with IDDM fail to produce insulin and, accordingly, are required to take daily insulin injections in order to stay alive. Many people are unaware that they have diabetes until they develop one or more of its life-threatening complications. Accordingly, much biomedical research has focussed on the cause and development of diabetes with the hope that having a better understanding of the disease will ultimately aid in earlier detection and/or better therapeutic treatments.
With regards to IDDM or Type 1 diabetes, three major theories have been advanced to account for the pathogenesis of the disease. The first is that IDDM is an inherited, or genetic disease. The second, that IDDM results from autoimmunity. The third theory states that IDDM is brought about by an environmental insult, presumably viral (Cotran (1989) Robbins Pathologic Basis of Disease 994-1005; Foster (1991) Harrison's Principles of Internal Medicine 1739-1759). Most agree however, that it is a combination of elements of all three theories that eventuates in IDDM, rather than each of the three acting independently in different individuals.
There is much evidence to support the theory that IDDM is an inherited disease. IDDM tends to aggregate in families, meaning that if one individual in a given family has the disease, each other member of the family has a greater chance of developing it. Certain HLA types, notably those of the D region of chromosome 6, carry an increased risk of IDDM (Cox et al. (1994) Diabetologia 37:500-503). Despite the presence of this genetic evidence, the facts remain that IDDM has a low prevalence of direct vertical transmission, and that the concordance rate of IDDM in monozygotic twins is only 20% (Cotran supra; Foster supra). This indicates that something more complex than simple Mendelian genetics is operating to cause the disease.
Several features of the pathogenesis of IDDM, resemble those of autoimmune diseases. Notably, patients newly diagnosed with IDDM have infiltration of the islets with activated T lymphocytes and antibodies directed against islet cell antigens, which are also present in the serum of non-diabetic siblings destined to develop the disease (Foster supra).
The third theory of the development of IDDM holds that diabetes results from environmental insult. Certain toxins can result in destruction of the pancreas, but the more likely offending agent is a virus. Infection with coxsackie B virus, congenital rubella, measles, mumps, cytomegalovirus, hepatitis, and infectious mononucleosis all carry increased risk of subsequent development of IDDM (Cotran supra; Foster supra). Pancreatic infection with one of these viral agents could bring about .beta.-cell destruction via direct inflammatory disruption, or by induction of an immune response (Foster supra).
More likely than one or the other of these theories explaining IDDM in a given individual is the combination of the three hypothesized causes participating in a sequence of events which results in the destruction of the .beta.-cell, and overt IDDM (Foster supra). A viral infection in a genetically predisposed individual could bring about an inappropriately large inflammatory response in the pancreas. Local inflammation can bring about the increased expression of novel MHC molecules on the surface of islet cells. In particular, the cytokines TNF-.alpha. and IL-1.beta., important players in the inflammatory response, have been shown to increase MHC expression on pancreatic .beta.-cells (Campbell and Harrison (1989) J. Cell Biochem. 40:57-66; Han et al. (1996) J. Autoimmunol. 9:331-339; Picarella et al. (1993) J. Immunol. 150:4136-4150; Ohashi et al. (1993) J. Immunol. 150:5185-5194). Novel MHC expression could bring about the eventual antibody formation and autoimmune destruction of the .beta.-cells, with IDDM as the result.
IDDM results from destruction of the insulin-producing .beta.-cells of the pancreatic islets. Without insulin, glucose is not effectively taken up into such metabolically active tissues as muscle, liver or adipose tissue. The results is hyperglycemia. Fasting blood glucose levels greater than 7.8 mM, or non-fasting levels greater than 11 mM result in the diagnosis of diabetes. Although levels of blood glucose are very high in uncontrolled diabetics, the body senses a "starved" state, and begins to release free fatty acids from adipose tissue. The blood levels of free fatty acids in the early stages of ketoacidosis can be in excess of 2 mM. Fatty acids are used as fuel by the liver and by muscle tissue because they can enter the cell freely, whereas glucose can no longer enter due to lack of insulin. The insulin deficiency, together with elevated free fatty acids stimulates gluconeogenesis, further exacerbating the hyperglycemia. The abundance of fatty acid oxidation occurring in the liver leads to an excess production of acetyle CoA. The excess acetyl CoA is converted into ketone bodies. Elevation and underutilization of ketone bodies and fatty acids produces a metabolic acidosis, termed diabetic ketoacidosis, which can progress to coma and death if not treated with insulin. The hyperglycemia present in IDDM is thought to contribute to the major pathologies associated with the disease, such as those found in the peripheral nerves, retina, kidney, and vasculature.
Although IDDM patients may have grossly elevated serum levels of free fatty acids, much less is known about how this may contribute to diabetic pathology than is known about hyperglycemia-related pathology. Even in non-ketotic states, IDDM patients have dyslipidemia, or elevated levels of fatty acid in the serum (Azad et al. (1994) Arch. Dis. Childhood 71:108-113). Following insulin-induced hypoglycemia, stimulation of diabetics with epinephrine results in increased free fatty acids greater than in controls subjected to the same maneuver (Bolinder et al. (1996) Diabetologia 39:845-853; Cohen et al. (1996) Am. J. Physiol. 271 :E284-293). Certain fatty acids have effects on various cells ranging from modulation of intracellular Ca.sup.2+ homeostatis (Deeney et al. (1992) J. Biol. Chem. 267:19840-19845, to activation of the nuclear transcription factor NF-kB, and modulation of gene expression (Prentki et al. (1997) Diabetologia 40 Suppl 2:S32-41; Prentki and Corkey (1996) Diabetes 45:273-283). Elevated extracellular free fatty acids result in increased cytosolic long chain CoA, the effects of which include modulating protein kinase C (PKC) activity, intracellular protein trafficking G-protein activity, endoplasmic reticulum (ER) Ca.sup.2+ -ATPase activity, expression of acetyl-CoA carboxylase and peroxisome proliferation (Prentki et al., supra; Prentki and Corkey (1996) supra; and Brun et al. (1996) 45:190-198).
Perhaps the most widely-accepted therapy for treating IDDM involves daily injection of insulin in combination with blood glucose monitoring and eating behavior modification, indirectly reducing undesirable secondary side effects and the risk of life-threatening complications. Moreover, alternative therapies including pancreas and islet transplantation, autoantigen-based therapies (e.g., glutamic acid decarboxylase (GAD) therapy), and .beta. cell-related peptide adjunctive therapies are being developed and tested. However, it is well-recognized that there remains a need for therapies that are more preventive in nature, in particular, therapies aimed at correcting the underlying abnormalities responsible for IDDM. Furthermore, there exists a need for new diagnostic tools aimed at identifying persons having or who are predisposed to the disease. In particular, there exists a need for methods of diagnosing persons at risk for developing IDDM, particularly during the long subclinical latency period associated with IDDM.